The present invention relates to pulse-width modulation systems for causing currents to flow through a load. More particularly, the present invention relates to switching circuits used to control current to the load.
Pulse-width modulation circuits are well known and are used commonly to control current to many types of loads, particularly inductive loads, including servo-amplifiers, brushless motors, other motion control systems and gradient coils in a magnetic resonance imaging (MRI) systems, energy storage devices, to name a few. The pulse-width modulation circuit includes a switching circuit to which the load is connected. The switching circuit, also know commonly as a xe2x80x9cbridgexe2x80x9d circuit, includes two branches or totems, wherein each branch includes two series connected switching elements. The load is connected across the midpoints of each of the series connected switches and a voltage is applied across each of the branches. By appropriately operating the switching devices, the voltage from the power supply can be placed across the load. As is well known, an average voltage can be obtained by modulating the duty cycle, or the time duration the bridge exists in each of its alternate conductive states. The average voltage is limited by the voltage potential of the power supply.
However, one drawback existing with the conventional bridge circuit used in the pulse-width modulation circuit is the limited voltage that can be applied to the load. In particular, the amplitude of the voltage potential is limited by the breakdown voltage of the switching devices used to construct the bridge. As a result of this voltage limitation, slew rates of an inductive load and the back EMF of a motor load are more difficult to overcome.
There is thus an on-going need for an improved switching circuit used to control current in a pulse-width modulation circuit. An improved bridge circuit that can apply large voltages across the load would be very beneficial.
A circuit for providing a current through a load includes a first bridge circuit connectable to a load. The bridge circuit selectively applies a voltage across the load. A second bridge circuit is connected to the first bridge circuit and is connectable to a power supply. The second bridge circuit selectively applies voltage to terminals of the first bridge circuit.
Another aspect of the present invention includes a circuit having a nested bridge circuit connectable to a load and a plurality of power sources. The nested bridge circuit comprises a plurality of switching devices. The switching devices are connected together so as to selectively apply a voltage across the load approximately equal to a maximum series connection of the plurality of voltage sources, while a maximum voltage across any one switching device does not exceed a maximum voltage of one of the plurality of power sources.
Yet another aspect of the present invention is a method of operating a pulse width modulating circuit. The pulse width modulating circuit comprises a first bridge circuit connected to a load and a second bridge circuit connected to the first bridge circuit and connected to a power supply. The method includes selectively applying at least two different voltage potentials to terminals of the first bridge through the second bridge; and operating the first bridge to apply an average voltage to the load.